This invention is generally directed to processes for the preparation of monodisperse polymeric particles and in situ toner compositions thereof, and more specifically the present invention is directed to starve fed or semi-batch emulsion polymerization processes for the preparation of monodisperse polymeric particles suitable for use in xerographic toner and liquid ink formulations. The polymeric particle compositions of the present invention in embodiments contain additionally optional colorants or magnetic pigment particles. In other embodiments, monodisperse polymeric particle compositions are prepared by processes of the present invention which have a core and shell structure comprised of different polymeric resins wherein the shell resins possess a higher glass transition (T.sub.g) temperature than the core resin and enables the separation and independent regulation of toner blocking and fusing functions. The monodisperse polymeric particle compositions of the present invention in embodiments, can be prepared by processes comprising performing a series of sequential emulsion polymerization reactions using starve feed emulsion polymerization. In embodiments, a first emulsion polymerization is accomplished to form very small submicron sized primary seed particles. The seed particles are then used as the starting point in subsequent growth steps to grow larger sized particles. The particle size distribution of successive generations of particles is essentially constant and monodisperse.
The resulting polymer particle compositions of the present invention are useful in a variety of applications, for example, for color or magnetic imaging and printing with dry and liquid developer compositions for electrophotography. These toner materials can be produced without the need for energy and capital intensive conventional multi-step post polymerization processing including for example compounding, jetting, and classification.
An important problem which has limited the preparation and maintenance of submicron polymeric materials by emulsion polymerization is the tendency of the initially formed small polymeric particles to agglomerate with one another which leads to undesirable broad particle size distributions and larger particle sizes.
In one prior approach directed at overcoming the aforementioned agglomeration problem there has been used protective colloids, for example, polymeric stabilizers or emulsifiers such as polyvinyl alcohol or complex polysaccarides such a TYLOSE.TM.. While polymeric stabilizers were effective in preventing agglomeration of the polymeric particles, the resulting particle size distributions were broad and the stabilizers are responsible for interfering with the charging and fusing properties, and humidity sensitivity of the resulting toner particles.
Polymerization techniques such as suspension and dispersion polymerization, or techniques such as precipitation or emulsion agglomeration are capable of generating homogeneous particles of a variety of copolymers. In the suspension polymerization process, an oil phase of monomer, colorant and initiator are emulsified into an aqueous surfactant solution to produce droplets which are subsequently heated to induce polymerization. In the dispersion polymerization process, an initially homogeneous mixture of monomer, initiator, steric stabilizer and solvent is heated to induce polymerization. The solvent is selected so that the steric stabilizer chains are soluble but the polymer being formed is insoluble, so particles with surface grafted steric stabilizer are formed. Although monodisperse particles can often be obtained, problems with this process include the need for a non-aqueous solvent and the inability to conveniently make copolymers while retaining the monodisperse size distribution. The product particles from precipitation processes are similar to those from dispersion polymerization, but they are formed by dissolving a polymer and graft or block stabilizer in a good solvent for both, then precipitating the polymer by adding a non-solvent for it which is a good solvent for one of the stabilizer blocks. Size distributions are extremely broad, however, and the reactor throughput or percent solids is low.
One major current use of emulsion polymers is in emulsion/agglomeration processes. In this process, submicron copolymer particles of 100 to about 200 nm are formed by traditional emulsion polymerization techniques, and then agglomerated together with pigment to form toner sized particles. However, the process has the following major disadvantages: some emulsions are difficult to agglomerate and do not coalesce into suitable toner particles, because of their charge properties, T.sub.g, or for other unknown reasons; the size distribution is not always narrow; and the process requires separate emulsion polymerization and agglomeration process steps.
The following references are mentioned:
U.S. Pat. No. 5,219,943, issued Jun. 15, 1993, to Akasaki et al., discloses a process for producing monodispersed fine particles of a vinyl polymer which comprises polymerizing monomers containing at least one vinyl monomer selected from the group consisting of aromatic vinyl compounds, acrylic esters, and a methacrylic ester in the presence of (1) a surface active agent represented by formula (I) wherein R.sub.1 and R.sub.2, which may be the same or different, each presents a substituted or unsubstituted alkyl group having from 4 to 8 carbon atoms, a substituted or unsubstituted phenyl group, or a substituted or unsubstituted cyclohexyl group and M represents an alkali metal, (2) from 10.sup.-3 to 10.sup.-1 mol/L of a persulfate as a polymerization initiator, and (3) from 10.sup.-4 to 10.sup.-2 mol/L of a divalent metal sulfate as an electrolyte. Monodispersed vinyl polymer fine particles having particle size of 1 micron or greater with a very narrow size distribution are obtained.
U.S. Pat. No. 4,703,090, issued to Ferraresi et al., discloses a method for preparing graft copolymers by means of a two-step polymerization in emulsion: in the first step a vinyl aromatic monomer and an acryl or methacryl monomer are grafted on a monomodal polybutadiene latex constituted of particles of diameter comprised within the range of from 800 to 2,500 Angstroms. The polymerization/grafting reaction is continued up to a 70-90% conversion, at which time a further portion of monomodal polybutadiene is added, together with further amounts of said monomers, always in emulsion.
U.S. Pat. No. 4,626,489, issued Dec. 2, 1986, to Hyosu discloses a polymerizable mixture containing a monomer, a polymerization initiator and a colorant which is subjected to suspension polymerization, and an additional monomer is adsorbed onto the resultant polymer particles and grown by the polymerization in the presence of an oil-soluble polymerization initiator to give a toner for development of electrostatic charges excellent in toner characteristics such as triboelectric charging characteristic, storage stability and free flowing property.
U.S. Pat. No. 5,273,824 issued Dec. 28, 1993 to Hoshino et al., discloses a cored multi-shell emulsion particle consisting of a core particle, a void layer existing in the exterior of the core particle and a shell layer, each of which has a diameter of .PHI., d and D, respectively, and the ratio of .PHI., d and D is in the following range EQU .PHI./D=0.1-0.6 EQU d/D=0.2=0.8 (d&gt;.PHI.)
The particle exhibits excellent hiding power, brightness, gloss and thermal insulation property as an organic pigment without impairing drying ability and strength of coated layer.
Other disclosures of emulsion polymerizations for forming small particles are generally known, but indications are that numerous problems exist, such as: only submicron particles, for example, less than 1.0 micron, can be formed; use of insufficient surfactant concentrations leads to emulsion or latex instability which in turn produces particle agglomeration; and 3) use of excessive surfactant concentrations leads to considerable secondary particle formation and broadened particle dispersities. The present intention overcomes or minimizes these and other problems by providing a means for controllably growing seed latex particles by regulating the ratio of water soluble to oil soluble free radical initiator in stepwise particle growth processes and by providing for in situ particle stabilization thereby minimizing or eliminating the use of exogenous surfactant stabilizers. The present invention also provides for, in embodiments, the capability to prepare toner sized particles which possess a encapsulated structure wherein theologically dissimilar resins are used for the particle core and encapsulating shell to control toner particle melt rheology(core) from toner blocking (shell) properties.
Other references of interest include: U.S. Pat. Nos. 4,996,127; 4,797,339; 4,876,313; and 4,983,488.
There remains a need for economic and convenient processes of obtaining very small particles with narrow particle size distribution properties, and more specifically micron and submicron polymeric particles, without the complications and disadvantages of the aforementioned prior art materials and processes. Further, there is a need for convenient means for preparing homogeneous polymeric particles and heterogeneous or encapsulated polymeric particles without the need of having to resort to intensive and expensive particle size reduction, comminution, or classification processes for obtaining clean, optionally dry and small polymeric particles, for example, from less than about 0.1 to about 10 microns in volume average diameter as determined by a scanning electron microscope, Coulter Counter Multisizer II or disc centrifuge particle size analyzer. Still further, there is a need for particle preparative processes that permit low cost, clean, and optionally dry micron and submicron polymeric particles that can be selected as liquid and dry electrophotographic developer compositions, carrier powder coatings, photoconductor pigment or resin coating suspensions, and as toner particles and toner additives for enhanced photoreceptor development and cleaning.